Advanced Power Management (APM) is an API that allows an operating system to interact with the BIOS to manage power. APM has been replaced by Advanced Configuration and Power Interface (ACPI). A device driver can be written to the APM spec to allow power management options. There are five power states for the system and four device power states. The CPU is controlled only by the BIOS to prevent crashes.
Advanced power management (APM) is an application programming interface (API) that allows an operating system running an IBM-compatible personal computer (PC) to interact with the basic input output system (BIOS) to obtain and adjust power management. It was created as a joint venture between Intel® and Microsoft® and was made publicly available in 1992. APM has largely been replaced by a more advanced management system known as Advanced Configuration and Power Interface (ACPI).
BIOS is a simple program that is hardcoded into all personal computers (PCs). It controls hardware at a basic level and can communicate with the operating system and any peripheral device, such as a hard drive. The operating system installs the APM API as part of its installation. The API is essentially software that facilitates communication between the operating system and the BIOS.
Whenever a device, such as a monitor or hard drive, is connected, a driver must be installed. The driver is software that allows the device to communicate with the operating system. The driver can be written to the APM spec so it can have power management options. An example would be a hard drive with an APM driver; the driver may allow the hard drive to be turned off when not in use.
There are essentially two states in advanced power management: system state and device power states. The system state can define five power states for the entire system. Full power on and off is the simplest; fully on means the system is on and no devices are in power saving mode, and off means the computer is off.
The other three advanced power management system states are APM Enabled, APM Stand, and APM Suspend. APM Enabled means that the computer is turned on and APM is available as needed. Standby means that most devices are in low-power mode, the central processing unit (CPU) is turned off or slowed down, and the system state is saved; in this state, the computer can start up quickly. APM Suspend means that the system state is saved but most of the devices are off; in this state, the computer takes longer to restart processing.
The device power states, which allow a device to use power-saving states, are Device On, Device Power Managed, Device Low Power, and Device Off. On and off mean the same as the system state. In Device Power Managed, the device is powered on but not all features are available. In the Device Low Power state, the device is not currently active, but the power connection is maintained so that it can be started or “woken up” quickly.
The one exception in the advanced power management environment is the CPU. Only the BIOS can control the CPU. A driver, whether through the system or through a device, can only contact the BIOS; can not give instructions to turn off. This is to protect the CPU from being crashed by a poorly written driver.
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